Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically, they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess the balance of properties required for this composite fabrication process. State-of-the-art epoxy/carbon fiber composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure. However, most epoxy formulations absorb moisture which reduces their higher temperature properties. As a result, they are not suitable for use at 270.degree. F. or greater in a moisture-saturated condition. Since most epoxy formulations used in prepreg are also brittle, the resulting composites have low toughness and poor impact resistance.
Polycyanurates, or cross-linked cyanate polymers, are prepared by the cyclopolymerization of aromatic cyanate esters. Such resins have long been known in the art, as have resinous compositions comprising polycyanurates and epoxy resins, shown, for example, in U.S. Pat. No. 3,562,214. Further blends and compositions that include thermoplastic resins as well as other thermoset resins are also known, as shown in U.S. Pat. Nos. 4,157,360 and 4,496,695. These mixed compositions have been developed to overcome the inherent limitations of most polycyanurates, including brittleness, poor processability and sensitivity toward wet or moist environmental conditions. However, the compositions presently available continue to exhibit some deficiency in moisture sensitivity and in processability, particularly when used in fabricating prepreg and composite structures using fibrous reinforcement materials.
There thus is a need for resin systems which afford composites with better toughness and which can retain a high level of properties at 270.degree. F. or greater in a moisture-saturated condition.